The Formation of Collagen Hydroxylysine Studied with Tritiated Lvsine*

Hydroxylysine was first isolated from gelatin (2) and probably does not occur in proteins of mammalian origin other than collagen (3). In rats, dietary lysine is an obligatory precursor of the hydroxylysine of collagen (4-7); when 14C-labeled lysine is either fed or given by injection it is incorporated into both the lysine and hydroxylysine of collagen, whereas labeled hydroxylysine is not incorporated int,o collagen at all (8). Since, very shortly after the injection of i4C-lysine into rats, the specific activity of hydroxylysine of skin collagen is exactly the same as that of collagen lysine, and since it remains the same for as long as 3 months (7), the hydroxylation of lysine must take place before it is incorporated into insoluble collagen. Recent reports from other laboratories (9, 10) make it seem probable that the hydroxylation of proline takes place after the amino acid is incorporated into a peptide chain, but before the peptide chain leaves the ribosome. It is possible that the hydroxylation of lysine takes place at the same time. The studies described in the present report were designed to aid in the understanding of the mechanism of the conversion of lysine to hydroxylysine. Conceivable mechanisms by which lysine could be hydroxylated are as follows. (a) Direct uptake of an oxygen atom on carbon 5 of the lysine chain